Copper containing sulphide phases present in controlled rolled niobium–titanium bearing high strength low alloy steels

Abstract

Complex copper bearing sulphide phases have been observed for the first time in controlled rolled niobium bearing high strength low alloy steels. Analytical electron microscopy studies were carried out on these copper bearing sulphides. The observations show that, in many cases, these particles have duplex sizes and are multiphase. The smaller particles have diameters from 0·2 to 0·7 μm with round or ellipsoidal disclike morphology, mainly present close to and/or within the pearlite banding where the segregation of the alloying elements is severe; they tend to be associated with other precipitates such as manganese sulphide, and titanium carbonitrides in the steels having titanium additions. The larger particles (>1 μm) occur occasionally, and are often associated with spinel MgO.Al2O3 and/or A12O3, Ti(C,N), MnS, and silica. These particles have complex chemical compositions with a ratio of (Cu + Mn)/S ranging from 0·77 to 1·42; their electron diffraction patterns lead to lattice parameters which are very close to either that of covellite (CuS) or digenite (Cu1·765S to Cu1·79S), The particles are probably the complexes of different phases, such as covellite phases and manganese sulphide, or digenite, covellite phases, and manganese sulphide. Some of the observed manganese sulphide particles also contain a small percentage of copper. With segregation of copper and sulphur in the as cast state, and in the presence of more strongly sulphide forming elements, such as manganese, the formation of copper bearing sulphide complexes can occur as the temperature decreases.MST/1603